In the last few decades a large number of antimicrobial systems have been developed that are based on the in-situ creation of metastable chlorous acid, under conditions where the chlorous acid, HClO2, represents a relatively small fraction of the total aqueous chlorite (ClO2−). The antimicrobially-effective chlorous acid systems function at pH values from about 3.5 down to about 2.5. The protic acid source to effect this conversion is generally an organic acid. These acidified chlorite compositions were first taught by Alliger in 1978 (U.S. Pat. No. 4,084,74) and in 1982 (U.S. Pat. No. 4,330,531), as germ-killing compositions with a broad-range of high activity against bacteria, fungi and viruses. In Alliger's compositions the acid activator, lactic acid, was deemed to be critical, and the quantity of lactic acid utilized had to represent at least about 15% by weight of the total amount of organic and inorganic acids present. Subsequently Tice, in 1986 (U.S. Pat. No. 4,585,482), disclosed long-active biocidal compositions in which sodium chlorite is slowly activated by the slow degradation of acid-generating polymers, such as poly(lactic acid). Several patents followed thereafter in which lactic acid, among other protic acids, was used in combination with chlorite salts for the purpose of skin disinfection, specifically as components of barrier formulations (U.S. Pat. No. 4,891,216), a genital herpes treatment (U.S. Pat. No. 4,956,184), or in anti-inflammatory formulations (U.S. Pat. No. 5,384,134).
Subsequent prior art taught the creation of a diverse range of chlorous acid compositions and their method of use. All but one (U.S. Pat. No. 5,820,822) were based on the use of protic acids, primarily organic acids, to transform chlorite to chlorous acid. These organic acids did not include lactic acid, as a result of Alliger's earlier disclosure of lactic/chlorite disinfection systems, wherein lactic acid was required to be at least 15% by weight of all acids present. These subsequent art disclosures are embodied in U.S. Pat. Nos. 4,986,990 (1991), U.S. Pat. No. 5,185,161 (1993) and RE36,064 (1999), all issued to Davidson and Kross. A process for using chlorous acid solutions to remove bacteria from poultry and other meats, issued to Kross (U.S. Pat. No. 5,389,390), included lactic acid as a prospective acidifying agent for the chlorite, but it was also disclosed that technical-grade lactic acid was inappropriate for such use because impurities in commercial sources of lactic acid caused the unwanted generation of chlorine dioxide (ClO2), and this was found to cause undesirable discoloration and bleaching of poultry skin This inventor has also noted that ClO2 causes similar, undesirable staining of protein deposits in soft contact lenses, following repeated use of the lenses and their subsequent disinfection in a ClO2 solution. The discoloration is most probably ascribable to the oxidative transformation, by ClO2, of labile amino acids (specifically tyrosine and tryptophan) in common proteins, to form colored materials (see Masschelein).
In the patent issued to Kross et al. (U.S. Pat. No. 5,100,652) “Disinfecting Oral Hygiene Compositions and Process for Using the Same,” which was based on chlorite solutions activated by an organic acid, lactic acid was specifically excluded from the list of organic acids that might be used in such solutions. Although this exclusion was necessary because of the earlier teachings of Alliger, lactic acid at that point was deemed to be an inappropriate mouth rinse acidifier, owing to its particularly undesirable sour taste. Thus its exclusion was not considered as limiting, when formulating effective, commercializable antimicrobial oral formulations. In the practice of the technology disclosed in Kross' oral hygiene U.S. Pat. No. 5,100,652, this same inventor has since learned that formulations which contain glycerin, particularly at levels above about 10% in the mixed composition, result in unacceptable staining of users' teeth. The use of such compounds, which contain vicinal hydroxy groups, were taught in that patent to be a means of releasing chlorine dioxide into the oral rinse for additional germicidal benefit. Nevertheless the adverse discoloration of teeth is now recognized to derive from chlorine dioxide, whether generated by glycerin-like compounds or the impurities associated with technical-grade lactic acid. At lower levels of ClO2 in oral rinse compositions the adverse staining effects take longer to notice, but for rinse products which are used twice-daily, for months and years, the increasing discoloration would dissuade consumers from further use. Thus the inappropriate taste of lactic acid as well as the propensity of normal, commercial lactic acid to cause unwanted tooth staining, would seem to argue strongly against its potential inclusion in oral rinse formulations.
Subsequent oral compositions based on acidified chlorite systems are those taught by Lukacovic et al. (U.S. Pat. No. 5,281,412), in which acidified chlorite solutions require a citrate ion source to reduce staining, and Witt et al. (U.S. Pat. Nos. 6,077,502 and 6,132,702) where the chlorite solutions are above pH 7, and contain effectively no ClO2 or chlorous acid.
Continued experiments intended to optimize the efficacy of oral rinse solutions has led this inventor to review and reconsider the potential contribution of lactic acid to these systems. This need was based on the fact that chlorous acid oral rinse systems are short lived, and subject to loss of activity upon salivary dilution and the resultant pH rise, while many organic acids will maintain some level of germ-killing activity in solutions with H+ concentrations one-tenth or less of their initial level in the chlorous acid oral rinse. The re-investigation of lactic acid specifically was driven by the fact that lactic acid is the most effective antimicrobial acid among those listed by the US Food & Drug Administration as Generally Recognized As Safe (GRAS) as food acidulants. The GRAS designation is accorded to materials which possess the highest recognized safety for food use; and GRAS acids, therefore, are the most appropriate to use for potentially-ingestible oral hygiene compositions. It is well recognized that lactic acid is more effective, for example, than malic, citric, tartaric, succinic, adipic, and fumaric acids, which are all GRAS acids. Among these acids, other than lactic, malic acid is the most active germicide, but when compared with lactic acid, it is significantly less effective. When lactic acid was compared with malic acid, on the basis of equimolar amounts of the unionized acid form, using an AOAC [Association of Official Analytical Chemists] Germicidal Test procedure, malic acid destroyed 1.45 logs10/ml organisms of an initial inoculum of E. coli of 7.63 logs10/ml, whereas lactic acid killed 4.15 logs10/ml of the same population. Numerically, after disinfection, there were ˜1500 times more residual E. coli organisms, per ml of solution, in the malic acid-treated suspension as there were in the lactic-acid treated suspension.
As already indicated, the impediments to the use of lactic acid in an oral rinse are its unpleasant taste and its tendency to trigger undesirable ClO2 formation in chlorite solutions, when included as the technical-grade material. The latter is generally ˜88% pure, and is the only form available commercially in appropriate bulk quantity, at a cost less that $1 per lb. This is in contrast to the limited quantities of “pure” (98%) lactic acid available from specialty chemical houses, at 10-times that price.
This invention is the result of efforts to capitalize on the well-recognized antimicrobial activity of lactic acid, as an auxiliary cidal agent to that of chlorous acid, while overcoming the limitations imposed by lactic acid's unpleasant taste as well as the ClO2-generating impurity effects of “technical-grade” lactic acid, the common commercial commodity. I have discovered that there is a unique “window” between the “0”, proscribed use level for lactic acid as the sole organic acid activator for chlorite in the Kross (U.S. Pat. No. 5,100,652) “Disinfecting Oral Hygiene Compositions. . . ” disclosure, and Alliger's teaching that lactic acid must represent at least 15% by weight of the acid(s) required to activate chlorite in his disinfecting compositions. I have found that the high germicidal potency of lactic acid allows for its effective use at levels low enough to avoid unpleasant tastes, and particularly, when included in the compositions in the manner taught in the following disclosure, effectively overcomes the staining potential engendered by ClO2-triggering impurities in technical-grade lactic acid. The lactic acid use level, in combination with at least one other organic acid activator, satisfies all of the technical and organoleptic requirements listed above, while uniquely fitting into the 0 to <15% concentration range window stipulated in the prior art.